The present invention is broadly related to structures and methods for coupling optical fibers to laser diodes and more particularly to thermally and mechanically stable high thermal conductivity structures and methods for coupling.
The three most common techniques used to align and attach single-mode optical fibers to high power laser diodes are molten solder fiber capture, omega yoke laser weld, and axial designs with separate X/Y and Z alignment laser welds. Techniques that use organic adhesives as a means of fiber attach are generally unsatisfactory because of thermal mismatch problems, mechanical stability problems, and laser reliability concerns. Although each method has been made to work they each suffer from significant deficiencies that negatively affect cost, performance, and reliability of the finished assembly.
Directing attention to FIG. 1, one common coupling and aligning technique, molten solder fiber capture, captures a metalized fiber in a molten drop of solder located close to the laser diode. When the solder is allowed to cool, the fiber is fixed in place. This technique suffers from a number of drawbacks. The solder solidification results in shrinkage that causes misalignment of the fiber from the laser diode. To correct the misalignment, the solder is re-melted, the fiber position is adjusted. The process is often inaccurate, and must be repeated numerous times to achieve good fiber alignment. Such an approach is costly as it requires an operator and the use of an alignment station during the time required for all of the solder re-flow operations. Another shortcoming of the molten solder capture technique is the significant potential for contamination of the laser diode facet from the molten solder pool and any contaminants that get onto the solder perform or substrate. Molten solder capture also presents a danger of overheating the laser diode, since the operation of the laser and the melting of the solder occur simultaneously. The solder typically used requires processing at temperatures above 300 Celsius. Metal coatings around the fiber and the metalization under the solder pool deteriorate with each melt cycle. After the solder solidifies and shrinks, the fiber is highly stressed at the two places where it emerges from the molten solder pool. This makes the fiber susceptible to breakage. The residual stresses in the solder are high enough that stress relaxation results in significant coupling efficiency changes. A high temperature oven bake is required to stabilize the assembly or significant coupling changes that are likely to result over extended periods. The technique is generally associated with planar package structures. The planar designs are torsionally weak and subject to optical misalignment from torsional forces. Strains that result from uneven package bolt down forces, or from imbalanced thermal stresses can lead to significant changes in fiber coupling efficiency. Planar structures are generally not rigid enough to allow significant path lengths or multiple optical elements. Alignment is usually performed with the assembly positioned within the package because of the sensitivity of the planar structure to stresses. This generally reduces access and visibility and reduces fiber alignment package yields while incurring high assembly labor and other costs.
Directing attention to FIG. 2, another known assembly and alignment method is the omega yoke technique that typically uses a laser welder to fix a fiber into an omega shaped metal support structure located directly in front of and in close proximity to the laser diode. This technique is commonly used for assembly of packages incorporating lensed fiber tips. The laser welded omega yoke technique uses a planar structure and thus suffers from the torsional rigidity problems of planar structures, the difficulty in allowing a long optical path or in incorporating additional optical elements, and the need to work inside of the package during alignment. Because laser welding occurs in close proximity to the unshielded laser facet there is the potential for laser ablated material and evaporated contaminants from the parts to be deposited on the laser facet. Because the weld process is not symmetric about the optical axis, each weld results in movement toward the mounting surface. The final alignment process is largely hit and miss, with a starting position above the optimum coupling position and each subsequent weld pulling the parts down and right or left. The cost of using the laser welded omega weld technique is high because an expensive combination laser welder and alignment station is tied up during the process. Also, the omega yoke is not as rigid as the molten solder method.
Directing attention to FIG. 3, the known technique of axial design with separate X/Y and Z laser welds requires the fiber alignment mechanism to be welded to the face of a hole bored along the laser optical axis. The fiber is pre-assembled into a weldable metallic tube. The tube slides axially through a bushing that abuts the face of the bored hole. Z axis alignment achieved by sliding the fiber metallic tube axially through the bushing hole. X/Y alignment is achieved by allowing the bushing to slide on the face of the bored hole. Two separate weld operations are required to permanently fix the fiber into alignment with the laser. It is customary to first align and weld the metallic fiber tube to the bushing to affix the Z axis. The X/Y plane is then realigned and the bushing laser welded to the face of the bore to fix the X/Y plane. There is weld distortion at each weld operation. The welds that attach the bushing to the bore face provide the only support to maintain fiber alignment. Weld shrinkage in this fillet weld often results in radial, axial, and angular misalignment. It is often necessary to resort to mechanical bending to try to achieve realignment. Laser hammering, the deliberate shrinkage of additionally applied asymmetric welds is used as a means to pull the structure back into alignment. Laser hammering is often touted as a benefit, but really is a sign of a design that has an unstable nature. The coupling value achieved before welding is seldom attained in the finished part. It should be noted that the assembly falls apart if released from the alignment fixture before welding. The parts have no inherent ability to remain aligned because all of their strength comes from the welds. It should also be noted that the operator, the alignment station, and the welder are tied up during both alignments as well as during laser hammering or mechanical bending. The reduced throughput of expensive laser alignment stations results in higher assembly costs.
Axial designs transfer heat from the laser diode to the package through a path that includes the same parts that provide mechanical support to the optical elements. Asymmetrical temperature gradients then occur which result in thermally induced losses of fiber coupling efficiency. Similarly, mechanical forces required to mount the aligned assembly to the package are often transmitted asymmetrically through optical support structures. The result is a completed assembly sensitive to temperature, package mounting stresses, and laser power dissipation.
In light of the shortcomings of the previous methods of aligning and attaching optical fibers to laser diodes, there remains a need for structure and method for coupling and aligning optical fibers to laser diodes that incur relatively low cost and yield higher connection efficiencies.
The present invention overcomes the shortcomings of the previous approaches to aligning and attaching optical fibers to laser diodes by providing a structure that allows both axial and radial alignment between a laser diode and optical fiber to be achieved in a single attachment process. The structure incorporates an optics tube, fiber holder, laser diode and heat sink. The fiber holder is affixed to the inside surface at one end of the optics tube and retains a segment of fiber in axial alignment with and close proximity to the laser diode. The heat sink is placed into the opposite end of the optics tube and serves to draw energy in the form of heat away from the laser diode. Optionally, a lens and lens holder may be incorporated into the structure between the laser diode and the fiber holder to focus laser energy on the end of the fiber. The laser diode is powered via an electrical lead that attaches to a metalized ceramic substrate located between the laser diode and the heat sink. The symmetrical design of the structure is rigid and substantially insensitive to thermal and mechanical stresses that cause misalignment in planar designs of similar dimensional proportion. The optics tube and heat sink elements of the structure are cylindrical in shape to allow high precision parts, as the heat sink and optics tube are easily turned and abrasively finished using standard machining equipment. The cylindrical shape of the heat sink also provides easy automation of a pre-tested semiconductor in fixturing with excellent thermal transfer. The high accuracy and finish of the cylindrical heat sink also allows excellent thermal transfer from the heat sink to auxiliary structures by mechanical means of fixation.
Because the heat sink and fiber holder elements are rigidly connected to each other via the optics tube, the heat sink car, be attached to other package structures without affecting the critical alignment between the laser diode and optical fiber. A plurality of welds can be used to connect the fiber holder and heat sink to the optics tube so that forces are transmitted through the structure through direct physical contact between the various elements. After alignment and before welding, elements can be released from an alignment fixture yet retain their alignment. By using mechanical attachment methods to secure the structure of the present invention to auxiliary structures, such as packages having thermoelectric cooler (TEC) elements, the present invention eliminates the need for large thermal excursions as required for soldering, or organic materials such as epoxy, as used in conventional methods of attaching a precision aligned laser and optical fiber to an auxiliary structure.